The present invention relates to a high-frequency tunable filter. It can be applied notably to the making of miniature filters designed for the high-frequency reception heads of VHF (very high frequency) RF receivers working in frequency hopping mode.
In RF receivers working in frequency hopping mode, there is a known way of placing a filter at the foot of the antenna to carry out the functions of filtering, selectivity and rejection of the image frequencies to enable the receiver to be tuned to the reception frequencies.
In the 30-88 MHZ or 68-150 MHZ frequency ranges, this filter is conventionally made in the form of two parallel oscillating circuits tuned by varicap (or varactor) diodes coupled to one another by mutual induction.
However, for the making of small-sized portable or hand-held RF sets, the amount of space taken up by these filters is very great because of the spacing necessary between the two oscillating circuits to achieve coupling by mutual induction. Furthermore, when these filters have to operate over extensive ranges of frequency, the adjusting of the spacing between the two oscillating circuits so that they remain in their critical coupling mode is difficult to achieve, and for very substantial frequency tuning ranges covering, for example, two to three octaves on frequency steps or stages that do not exceed 10 ms, this coupling mode does not allow the coupling circuits to remain in a critical coupling state throughout the frequency range.